REVIEW—ERUPTIVE ROCKS OF BRENT TOR. 17 
The Eruptive Rocks of Brent Tor and its Neighbourhood. By Franx 
Ruttey, F.G.8., Geological Survey Memoir. Price 15s. 6d. 1878. 
Surety the memoirs of the Geological Survey must be of a very bashful 
and retiring nature, for the amount of ‘imtroduction” which they 
require on making their appearance in public is something remarkable. 
Here we have a small pamphlet of fifty-five pages in a paper cover. The 
first page, in large print, is occupied by Professor Ramsay, the Director- 
General of the Survey, with some general introductory remarks. Then, 
in smaller letters, Mr. Bristow, the Senior Director, paraphrases 
Professor Ramsay—or, perhaps, we ought rather to say the matter is 
vice versa, as Mr. Bristow’s ‘‘ Notice” is dated a week the earlier of the 
two. lLastly,in still smaller type, as becomes his junior official condition, 
the author’s ‘‘ Preface” appears. 
The first chapter consists of some very useful introductory remarks 
on the use of the microscope in petrographical research. In connection 
with this matter, it is greatly to be wished that some competent worker, 
such as Mr. Rutley himself or Mr. Sorby, Mr. Allport or Professor 
Bonney, would write us an English text-book on the subject. At present 
we are mainly dependent on the Germans, Rosenbusch and Zirkel. 
The next chapter gives us a description of Brent Tor and its 
neighbourhood. It is situated on the west of Dartmoor, between the 
Rivers Tamar and Tavy. This district was geologically mapped by 
the founder of the survey, the late Sir Henry de la Beche, in 1839. One 
would have thought that a fresh map of the district on the 6-inch scale 
would have been undertaken prior to the publication of this Memoir ; 
and there can be no doubt that, if Mr. Rutley had had such a map to aid 
him, his labours would have gained greatly in precision and certainty. 
In Part II. we find the result of the microscopic examination of 
twenty-seven rock specimens, thirteen of which are illustrated by very 
carefully executed coloured figures; this is decidedly a valuable contri- 
bution to tbe subject. Finaily, the author states the ‘ deductions” 
which, in his opinion, are to be drawn from the mode of occurrence of 
the rocks in the field, together with the evidence they yield under the 
microscope. He agrees with De la Beche in considering Brent Tor a 
fragment of the nucleus of an old volcano, while the schistose ash beds 
of the neighbourhood possibly emanated from it, both being of carbon- 
iferous age. These beds owe their preservation to their being on the 
downthrow side of a fault ranging from N.W. to S.E., along the eastern 
edge of the Tor. From a reference on page 78, this work appears to have 
been written nearly three years ago, although only now published; and 
this delay of publication also applies to most of the other Memoirs of 
the Geologica] Survey, a delay which is neither just to the authors nor to 
the public. 
Mr. Rutley’s abilities with the pencil are well known, and this book 
is illustrated with six plates and ten woodcuts, which deserve much 
better paper than that upon which they are printed. The four plates of 
chromolithographs of microscopic sections we have already referred to. 
W. J. H. 
D 
